Regular use of chemicals to control unwanted organisms can select for drug resistant strains. This has occurred in many species of economically important pests. The development of drug resistance necessitates a continuing search for new control agents having different modes of action.
Many hundreds of strains of Bacillus thuringiensis (B.t.) produce insecticidal toxins designated as delta endotoxins. They are synthesized by sporulating B.t. cells. When toxin is ingested by a susceptible insect, the cells of the gut epithelium are destroyed.
The reported activity spectrum of B.t. covers insect species within the orders Lepidoptera and Coleoptera, many of which are major pests in agriculture and forestry. The activity spectrum also includes the insect order Diptera, which includes mosquitoes and black flies. See Couch, T. L. (1980) "Mosquito Pathogenicity of Bacillus thuringiensis var. israelensis," Developments in Industrial Microbiology 22:61-76; Beegle, C. C. (1978) "Use of Entomogenous Bacteria in Agroecosystems," Developments in Industrial Microbiology 20:97-104. Two varieties of B.t. known to kill mosquitos and blackflies are B.t. israelensis (B.t.i.) (Goldberg, L. J., J. Margalit [1977] Mosquito News 37:355-358) and B.t. morrisoni (B.t.m.) (Padua, L. E., M. Ohba, K. Aizawa [1984] J. Invertebrate Pathology 44:12-17). These are not harmful to non-target organisms (Mulla, M. S., B. A. Federici, H. A. Darwazeh [1982] Environmental Entomology 11:788-795), and play an important role in the integrated management of dipteran pests. They are safe to use in urban areas, and can be used in aquatic environments without harm to other species. Dipteran insects are serious nuisances as well as being vectors of many serious human and animal diseases such as malaria, onchocerciasis, equine encephalitis, and dog heartworm.
A small number of research articles have been published about the effects of .delta.-endotoxins from B. thuringiensis species on the viability of nematode eggs. Bottjer, Bone, and Gill (Experimental Parasitology 60:239-244, 1985) have reported that B.t. kurstaki and B.t. israelensis were toxic in vitro to eggs of the nematode Trichostrongylus colubriformis. In addition, 28 other B.t. strains were tested with widely variable toxicities. Ignoffo and Dropkin (Ignoffo, C. M. and Dropkin, V. H. [1977] J. Kans. Entomol. Soc. 50:394-398) have reported that the thermostable toxin from Bacillus thuringiensis (beta exotoxin) was active against certain nematodes. Beta exotoxin is a generalized cytotoxic agent with little or no specificity. Also, H. Ciordia and W. E. Bizzell (Jour. of Parasitology 47:41 [abstract] 1961) gave a preliminary report on the effects of B. thuringiensis on some cattle nematodes.
Sheep lice belong to the order of insects known as biting lice [Phthiraptera, suborder Mallophaga]. Some of the most common and economically important species of lice on mammals belong to the genus Damilinia which have sheep, cattle, goats and equines as their hosts. The general life span of these insects is about a month during which time the female lays 200-300 operculate eggs ("nits"). These are usually whitish in color and glued to the hair. There is no true metamorphosis but after about three moults a fully grown adult is present. The Mallophagans are equipped for biting and chewing. They ingest the outer layers of the hair shafts dermal scales, and blood scabs on mammals.
The genus Damilinia contains species which are capable of rapid population expansion by changing to asexual reproduction by parthenogenesis. Other important genera in Mallophaga include Felicola which are parasites on cats, and Trichodectes and Heterodoxus both of which utilize dogs as their hosts.
A closely related suborder is Anoplura which are sucking lice occuring solely on mammalian hosts. They have very similar life cycles but have piercing mouthparts allowing them to feed on the blood of cattle, pigs, equines, goats, and dogs. Economic genera within this suborder include Haematopinus, Linognathus, and Solenopotes.
Damilinia ovis, the biting louse of sheep, is more active than Linognathus and roams throughout the wool over the sheep's body. Damilinia can produce great irritation causing the sheep to become restless and have their grazing interrupted. Their condition deteriorates and in response to the irritation, the sheep rub against posts and wires which damage the fleece and also causes wool loss. When these lice bite there is an exudate of serum from the damaged skin on which the lice also feed. Heavy feeding increases the amount of exudate which causes two problems: the wool matts, reducing the value of the wool clipped and sheep blowflies, attracted by the damaged fleece and skin and louse feces, can cause strikes placing the animal at even higher risk.
Lice are normally treated by dipping, spraying, or drenching with insecticides containing synthetic chemical pesticides such as pyrethroids or avermectins.
At the present time there is a need to have more effective means to control lice that cause considerable damage to susceptible hosts. Advantageously, such effective means would employ biological agents.